The electrolysis of water is presently used as a source of hydrogen in, for example, hydrogen filling stations and chemical plants, and as a way of storing excess renewable energy using a technology called “power to gas”.
There are two main types of low temperature electrolyzers: proton exchange membrane (PEM) electrolyzers and alkaline water electrolyzers. PEM electrolyzers are able to operate at current densities around 1 A/cm2 at cell voltages less than 2 V, and can be turned on and off quickly. Unfortunately, PEM electrolyzers typically require the use of precious metal catalysts, such as platinum or iridium. This is a significant economic limitation.
In contrast, the present generation of alkaline water electrolyzers do not require the use of precious metals; instead, base metal catalysts can be used. Unfortunately, the present generation of alkaline water electrolyzers typically operate at lower current densities than PEM electrolyzers. This raises the capital equipment cost.
For example, U.S. Pat. No. 4,445,994 notes that alkaline water electrolyzers operate at 200-300 mA/cm2 (0.2-0.3 A/cm2) at cell potentials of 1.8-2 V. The '994 patent also indicates that currents of 1 A/cm2 can be obtained by increasing the cell temperature to 110° C., but as pointed out in U.S. patent application Ser. No. 15/029,952 (published as US2016/0237578), the upper temperature limit of a practical alkaline water electrolyzer is 80-90° C., since excessive corrosion is observed at above 80-90° C. in these high pH systems (for example, 1 M KOH electrolyte.) Recently, U.S. patent application Ser. No. 15/103,924 (published as US2016/0312371) disclosed an improved electrolyzer design that can obtain 40 A/dm2=0.4 A/cm2 at 80° C. and 1.9V. This is a significant improvement, but still below the performance of a PEM electrolyzer.
Anion Exchange Membrane (AEM) water electrolyzers have also been started to be discussed in the literature as replacements for conventional alkaline water electrolyzers. A recent review by Vincent and Bessarabov (Renewable and Sustainable Energy Reviews, volume 81, pages 1690-1704 (2018)) states that “few research articles (less than 20) have been published on AEM water electrolysis.”